<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="../js/jquery-1.11.3.js"></script>
    <script>
        $(function(){
            $('input:button').click(function(){
               alert('aa')
                $('#username').trigger('focus')
            })
//            $('input').click();
//            $('#username').focus();
        })
    </script>
</head>
<body>
<input type="text" id="username">
<input type="button" value="点我">
<div>
    <h1>aaasd</h1>
    <p>aaasd</p>
</div>

</body>
</html>